P0299
PowertrainTurbocharger/Supercharger "A" Underboost Condition
Your turbocharger isn't producing the boost pressure the ECU expects. The engine is therefore underpowered, and on diesel engines particularly the car often drops into limp mode to protect itself.
ⓘ Information only. This page provides general educational information about fault code P0299. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0299 mean?
P0299 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Turbocharger/Supercharger "A" Underboost Condition.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Sudden, dramatic loss of power, you put your foot down and almost nothing happens
- • Limp mode, the car will run but won't go above about 3,000 rpm or 50-60 mph
- • Engine warning light, sometimes accompanied by glow plug or DPF lights on a diesel
- • Sometimes a faint hissing or whistling under load (that's a split intercooler hose if it's audible)
- • Black smoke on a diesel under acceleration, less common with modern engines
Possible causes
Causes commonly associated with P0299, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Split intercooler hose, the single most common cause on diesels. Usually the lower one that runs underneath, where stones and heat eventually crack it
- 2. Variable-vane turbo (VNT/VGT) sticking from carbon buildup. Almost universal on EU-emission diesels past 80,000 miles
- 3. Boost pressure sensor faulty or with oily contamination
- 4. Vacuum or electrical actuator on the turbo failed (N75 valve on VAG, equivalent on others)
- 5. DPF heavily blocked, creating exhaust back-pressure that strangles the turbo
- 6. Wastegate stuck open on petrol turbos
- 7. Failed turbo (rare unless there's audible noise or visible oil)
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Bonnet up, engine off, get under and visually inspect every intercooler hose. Squeeze them, look for cracks, oil residue at clamps, or hoses that have popped off entirely. This finds the fault on a fair share of P0299 cases
- 2. Pull live data, look at requested boost vs actual boost. A clean split between them confirms the turbo isn't responding to commands
- 3. On a VAG diesel, check the N75 valve operation, swap it with a known-good one if you can
- 4. Inspect the boost sensor for oily contamination from a leaking turbo
- 5. Carbon-clean test: with a manual variable-vane actuator, check whether the vanes move freely through their full range. Stuck vanes from carbon are extremely common and sometimes free up with a chemical clean rather than a turbo replacement
Common questions about P0299
Why does my diesel keep going into limp mode? +
Limp mode is the ECU protecting the engine when boost is dangerously out of expected range. It restricts revs and fuel to keep the turbo from over-spinning or the engine from running lean. Once you've fixed the cause and cleared the code, normal driving resumes.
Can a stuck variable-vane turbo really be cleaned? +
Yes, often. The variable vanes that control boost in a VNT turbo seize up with carbon over time, especially on cars that mostly do short journeys. A chemical clean (workshop service) restores movement in many cases. If the vanes have actually broken or the bearings are gone, replacement is the only fix.
Is the car safe to drive home in limp mode? +
Drivability is fine for short distances at reduced power, the engine isn't at risk so long as you're not flogging it. The bigger risk is being unable to safely overtake or get up to motorway speeds. Most drivers can manage a limp-home journey to a garage. Always confirm with a mechanic.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →